1. Field of the Invention
The present invention relates to a radiation measurement apparatus, and particularly relates to a radiation measurement apparatus that measures a radiation dose per unit time by measuring a current signal which is output from a radiation detector.
2. Description of the Background Art
Radiation measurement apparatuses which are installed in a nuclear power plant, nuclear fuel reprocessing facilities, radiation utilization facilities and the like, and in the vicinity of these facilities are required to cover a wide measurement range from a normal radiation level to a radiation level at which an accident is taken into consideration, in the measurement of a dosage rate or the like. For this reason, when an ionization chamber is used, for example, as a radiation detector, a fine and wide range of current having an order of 10−14 A (ampere) to 10−7 A which is generated as a result of the action of radiation on the ionization chamber is required to be measured with a good degree of accuracy.
As a micro-current measurement apparatus that measures such a micro-current, an apparatus is used which includes current integration means for repeating an operation in which an input current is integrated up to a predetermined value and is output, counting means for counting the number of repetitions per setting time performed by the current integration means, current integration value detection means for detecting a current integration value in the current integration means when the setting time has elapsed, and arithmetic operation means for calculating a current value of the input current on the basis of a characteristic value of the current integration means, the setting time, and the current integration value which is detected by the current integration value detection means, wherein electric charge of the input current is integrated and converted into a voltage, electric charge of a predetermined value is discharged when a sawtooth-wave output voltage reaches a voltage of a predetermined value, and is converted into a sawtooth pulse of a repetitive frequency which is proportional to the current value, a fractional count value obtained on the basis of a voltage value of a sawtooth wave-shaped pulse which is measured at a point of time when the setting time has elapsed is added to a count value obtained by counting the number of square-wave pulses in the setting time, to set the resultant value to a real count value, and a micro-current is measured on the basis of the real count value (see, for example, Patent Document 1).
In addition, there is a radiation photon counting system that performs highly-sensitive measurement in a region has a lower radiation intensity than in a current integration system of Patent Document 1. In this system, electron-hole pairs are generated at a position in which radiation photons within a radiation detector are absorbed, and transfer toward both electrodes of the detector to induce electric charge in the electrodes, whereby a current pulse is generated. The current pulse is converted into a voltage pulse which is proportional to energy of the absorbed radiation photons, a reverse bias voltage is applied to a Schottky junction in order to constitute a detector for the radiation photons, and the junction is used as a layer sensitive to radiation by depletion. In a radiation detection apparatus based on this radiation photon counting system, a technique is proposed in which a current supply circuit that supplies a current to a current input terminal of an electric charge amplifier is provided, a reverse bias leakage current flowing into the radiation detector does not flow through a feedback resistor of the electric charge amplifier by predetermined value or greater, a bias voltage is increased little by little while monitoring an output voltage of the electric charge amplifier, and a stable operation is performed (see, for example, Patent Document 2).
Further, as a technique for measuring a detection current which is obtained by detecting radiation with a high level of accuracy, particularly, a technique for correcting an error component associated with a leakage current of an element for measuring the detection current, a configuration is proposed in which an element for a leakage current having the same characteristics as the characteristics of an element for a detection current is disposed so as to have the same temperature conditions with respect to a measurement error associated with a change in temperature, particularly, when a micro-current is measured, and compensation is performed by subtracting a measurement value based on the element for a leakage current from a measurement value based on the element for a detection current (see, for example, Patent Document 3).
[Patent Document 1] JP-A-61-83967
[Patent Document 2] JP-A-3-72289
[Patent Document 3] JP-A-2008-145264
In a micro-current measurement apparatus of the related art, when the micro-current measurement apparatus is applied to a radiation measurement apparatus, a leakage current of the micro-current measurement apparatus is superimposed on a current signal of the radiation detector in an input of the micro-current measurement apparatus, and the leakage current changes depending on a temperature. Therefore, in the vicinity of the lower limit of a measurement range having an order of 10−14 A, the leakage current becomes relatively larger with respect to the current signal from the radiation detector, and thus there is a problem in that a measurement error may increase when the measurement of a low radiation dose is performed in a usage environment having a large change in temperature.
In the micro-current measurement apparatus, the maximum value of annual environmental temperature is taken into consideration in order to reduce the influence of temperature characteristics, and a heater is delicately controlled so as to make temperature constant at the maximum temperature. However, in a case of outdoor installation, in an overcoat that having the ionization chamber and the micro-current measurement apparatus housed therein, heat infiltrating from an insolation surface and heat generated inside the apparatus are radiated from a shaded surface, and thus internal temperature distribution subtly changes depending on seasons, the weather, and surrounding environmental conditions. There is a problem in that indication may fluctuate by the action on this subtle change, particularly, the temperature characteristics of a backward leakage current of a diode of a discharge circuit.
In Patent Document 3 described above, a proposal is made for correcting an error associated with the leakage current due to a change in temperature, but the element for a leakage current having the same characteristics as the characteristics of the element for a detection current is required to be disposed so as to have the same temperature conditions. For this reason, there is a problem in that actual circuit characteristics may be required to be the same as each other, and a work for selecting the characteristics from a large number of components so as to have the same characteristics may be inevitably needed.